(1) Field of the Invention
The present invention relates to a solid-state imaging device, and particularly relates to a back-illuminated MOS solid-state imaging device.
(2) Description of the Related Art
In recent years, in a solid-state imaging device such as a CCD image sensor and a MOS image sensor, the need for further reduction in pixel area has been growing to meet the need for further increase in number of pixels. Along with this, in a surface-illuminated solid-state imaging device which includes electrodes and lines on a substrate surface in an imaging region and receives incident light from a surface side of the substrate, it becomes more difficult to achieve sufficient light collection properties for collecting the light onto a photoelectric conversion unit (photodiode). As a means to solve this problem, a back-illuminated solid-state imaging device has been suggested whose light collection properties are enhanced by causing photoelectric conversion inside the substrate through reception of incident light from a back side of a substrate in which no line or electrode is provided (for example, see: T. Joy, et al., “Development of a Production-Ready, Back-illuminated CMOS Image Sensor with Small Pixels”, IEDM2007, pp. 1007-1010; and Japanese Unexamined Patent Application Publication NO. 2006-49338).
On the other hand, when the solid-state imaging device is irradiated with light of high intensity, a large amount of charge is generated in the photodiode, and the amount of the generated charge exceeds an amount of charge that the photodiode can accumulate. In this case, the charge leaks into an adjacent photodiode, causing output of a signal that has not originated from an imaging object. This phenomenon is generally called blooming. A generally-known means for suppressing the blooming is an overflow drain configuration. FIGS. 10A and 10B each show an example of the overflow drain configuration adopted for the back-illuminated MOS solid-state imaging device.
FIG. 10A is a cross-sectional view showing an example of a pixel configuration of a solid-state imaging device 10 which is a conventional back-illuminated MOS solid-state imaging device. The solid-state imaging device 10 includes an n-type photoelectric conversion unit (photodiode) 21 formed inside a p-type semiconductor substrate, and an element separation region (not shown). In addition, the solid-state imaging device 10 includes: a gate oxide film (not shown) of a MOS transistor, gate electrodes 31 and 32, a contact (not shown), and a wiring layer 40 which are formed on a second main surface 16 (top side) of the semiconductor substrate 30. In addition, the solid-state imaging device 10 includes a color filter 64 and an on-chip lens 65 which are formed above a first main surface 15 (back side) of the semiconductor substrate 30.
The overflow drain configuration of such a back-illuminated solid-state imaging device 10 will be described with reference to FIG. 10B. FIG. 10B is a cross-sectional view mainly showing, of the pixel cross-section shown in FIG. 10A, a configuration of a main part of an inside of the semiconductor substrate 30. As shown in FIG. 10B, the following elements are formed on the semiconductor substrate 30: an n-type floating diffusion region 33 for converting, into voltage, the charge read from the photodiode 21 of n-type; and an overflow barrier region 36 which functions, when the photodiode 21 accumulates an excess charge, as a path for discharging the excess charge to the floating diffusion region 33. Here, the solid-state imaging device 10 is so designed that the overflow barrier region 36 has a higher potential than a p-type region (semiconductor substrate 30) around the overflow barrier region 36.
The floating diffusion region 33 is connected to a transfer transistor 22 and a reset transistor which discharges the charge from the floating diffusion region 33. The solid-state imaging device 10 discharges the charge to a power supply by turning on (turning into an ON-state) the reset transistor after converting the charge into voltage (after a reading operation). Thus, in the conventional solid-state imaging device 10, by keeping the reset transistor in an ON-state except when reading the charge, the excess charge which is an excessively-accumulated amount of charge is discharged from the overflow barrier region 36 via the floating diffusion region 33. This prevents the excess charge from flowing into the photodiode and so on of another pixel.